This invention relates to a method and apparatus for operating a PCM time division multiplex communication network having digital subscriber stations. Pursuant to the invention, the subscriber stations accept PCM data sent from a central processing office which designates the particular subscriber station intended to receive the PCM data. The designation of a particular subscriber station is accomplished by a special code word sent from the central office and prefixed to the outgoing PCM data.
This special code word may be the address of the subscriber station (for example, P 2 341 115) or--in the case of a switching process wherein the subscriber stations are connected in a defined sequence to a common time division multiplex line and wherein the PCM data are transmitted from the central office in the same sequence in which the subscriber stations are located on the time division line--it may be a head code word which, upon being received for the first time within a sampling pulse frame, results in the acceptance of the PCM data.
Under both these methods, the special code word is used to introduce an information signal to the subscriber stations. This information signal is a bit group which normally comprises the PCM data transmitted from the central office and a group of unmodulated bits likewise transmitted from the central office. This group of unmodulated bits, which may be provided before or after the PCM data, normally serves as a carrier bit group for the modulators of the subscriber stations. By using these unmodulated bits in conjunction with the PCM data, no special synchronization is required to operate the modulators in the subscriber stations.
This method of incorporating unmodulated bits in the information signal together with the PCM data requires use of broadband transmission media. For example, the transmission media might be glass fiber paths for use with information carrying laser beams. The use of such transmission media usually solves the bandwidth requirements imposed by this technique of transferring an unmodulated bit group as long as only one type of data is to be transferred (e.g. telephone data). If, however, in one and the same system, additional data other than telephone data is to be transmitted, such as, for example, broadcast data or television data in conjunction with the video, traffic may be impeded. For these multiple data systems, a bottleneck is likely to occur if a large number of subscriber stations are connected. This bottleneck will occur even if broadband transmission media such as glass fiber paths are used.